The present invention relates to a sensitivity compensating method for a solid-state image pickup element used in a camera such as an electronic still camera or a video camera. More particularly, the invention relates to such a method for compensating for the sensitivity of a solid-state image pickup element whose sensitivity drops as the shutter speed is increased, even if the exposure level of the image pickup element is maintained at a constant value by controlling the aperture setting or shutter speed.
A conventional electrical still camera is arranged as shown in FIG. 1. The optical image of a photographed object passes through a photographing lens 1, an aperture control iris 2, and a shutter 3, and is focused upon a CCD- or MOS-type solid-state image pickup element 4. In the image pickup element 4, the optical image of the object is subjected to photoelectric conversion on a pixel basis and an electrical signal is outputted therefrom.
A synchronizing signal generating circuit 5 generates various control signals used to control the operations of the electronic still camera. A driving circuit 6 connected to the synchronizing signal generating circuit 5 generates a driving signal applied to the image pickup element 4. Upon receipt of the driving signal, the image pickup element 4 outputs electrical signals with a predetermined timing on a pixel basis. Since color filters are provided on the image pickup element, color signals for each of red (R), green (G) and blue (B) are outputted from the image pickup element.
A signal processing circuit 7 receives the output of the image pickup element 4 and carries out preprocessing of the signals representative of R, G and B, and converts the signals into luminance signals and color-difference signals using a matrix circuit provided within the signal processing circuit 7. The output of the signal processing circuit 7 is supplied to a recording unit 8 in which the luminance signals and the color-difference signals are subjected to FM modulation. The modulated signals are recorded on a magnetic recording disk or the like.
A light measuring circuit 9 measures the amount of light received from the object before a photograph is taken and supplies the measured data to an exposure controlling circuit 10. A shutter/aperture driving circuit 11 controls the iris 2 and the shutter speed in response to an instruction received from the exposure controlling circuit 10 so that the amount of exposure of the image pickup element 4 is maintained at an optimum constant value.
A memory circuit 12, composed of semiconductor memory elements, such as a RAM or ROM, stores adjusting data for optimum stop aperture and shutter speed values with respect to the light level of the object as measured in the light measuring circuit 9. The optimum aperture setting and shutter speed are provided to maintain the exposure level at a constant value. More specifically, the exposure controlling circuit 10, supplied with the output of the light measuring circuit 9, determines the light level (illumination level) of the object based on this data. Adjusting data values for the aperture and the shutter speed corresponding to the light level of the object are read from the memory circuit 12 and then supplied to the shutter/aperture driving circuit 11 where the aperture setting and shutter speed are determined so that optimum exposure conditions are attained at the time of photographing.
In such an electronic still camera, both the aperture setting and the shutter speed are simultaneously adjusted in accordance with the amount of light being measured and also in accordance with whether priority is to be given to the aperture setting or to the shutter speed. In an electronic still camera having the abovedescribed arrangement, and particularly in such a camera using a solid-state image pickup element with a narrow range of acceptable exposure levels, it is most important to control the exposure level.
Although it may seem that ideal photographing conditions can be maintained if control is effected so that a constant amount of light strikes the image pickup element by adjusting the aperture setting and the shutter speed, a problem arises in practice in that the level of the electrical signal outputted from the image pickup element gradually drops as the shutter speed increases, even if the exposure level of the image pickup element is maintained at a constant value.
FIG. 2A is a graph representing the relation between the output level of the image pickup element 4 and the shutter speed for a constant amount of light on the image pickup element. In this graph, the output level of the image pickup element 4 at a shutter speed of 1/60 seconds is normalized to 100%. According to the measured values as represented in the graph, the lowering of the output level of the image pickup element is not negligible when the shutter speed is faster than 1/1000 sec. This causes a degradation in the picture image in an electronic still camera using such an image pickup element.
A further problem arises in that the signal level drop-off phenomenon dependent on shutter speed also occurs for each hue at a different ratio. FIG. 2B shows red and blue characteristic curves. When the shutter speed is faster than 1/000 sec, the drop of the signal levels for both red and blue become significant. This causes a loss of color balance, and thus a high quality picture image cannot be obtained.